Regarding your question: "What is the correct sexual etiquette for a strict Grhastha; and what is spiritual family planning?" Unless one wants to beget a child, there should not be any sex life. The best thing is to forget sex, but it is not possible immediately or all of a sudden, especially in the Western countries where sex life is so liberal. So under the circumstances, one should try to have sex life only for children, not for any other purpose. Spiritual family planning is that one should be determined to train up children in Krishna Consciousness. According to Bhagavata, the spiritual family planning is that one should not become a father or one should not become a mother, unless he is able to maintain their children to the extent of liberation. It is the duty of the parents to see that the children are growing luxuriantly not only materially, but spiritually also. So spiritual training should be given from the very beginning. Kaumaram acaret prajna—In the Srimad-Bhagavatam, Prahlada Maharaja has instructed that spiritual consciousness or Krishna Consciousness should be taught to the children from the very beginning as they are given education from early childhood.
The next question, "will you please explain about the 28th mahayuga of this Manu, in which the treta and Dvapara yugas are reversed?" The 28th mahayuga means that in Brahma's one day, there are 14 Manus. And each Manu's life is the duration of 71 mahayugas. And one mahayuga means 4 yugas combined. The duration of Sattva yuga is about 18 hundred thousands of years. And the duration of Treta yuga is about 12 hundred thousands of years.